1. Field of the Invention
The present invention relates to a low noise amplifier (LNA), and more particularly, to an LNA with variable gain, common gate bias circuit and auxiliary matching mechanism.
2. Description of the Prior Art
Low noise amplifiers (LNAs) are essential building blocks in wireless receivers, especially for those systems seeking more compact and power-efficient performance. To achieve a compact solution, a wireless GSM receiver must utilize a highly linear LNA to avoid SAW (surface acoustic wave) filters. In order to obtain high linearity, conventional LNAs utilize inductors for loading and degeneration to achieve linearity and matching simultaneously, where the inductors are large in size and impossible to shrink no matter how advanced the semiconductor process.
In addition, some conventional receivers require a variable gain range for wideband applications. In this case, conventional LNAs usually utilize a shunt-shunt feedback to achieve a variable gain range in exchange of a maximum gain. The amount of the feedback signal has a great influence on input matching and linearity, meaning that certain issues may occur when the conventional LNAs alter their gain values. Moreover, the shunt-shunt feedback is based on a voltage mode operation, which is not suitable for a SAWless receiver due to linearity issues.